Crystal structure and characterization of a novel organic optical crystal: 2-Aminopyridinium trichloroacetate

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Abstract

2-Aminopyridinium trichloroacetate, a novel organic optical material has been synthesized and crystals were grown from aqueous solution employing the technique of controlled evaporation. 2-Aminopyridinium trichloroacetate crystallizes in monoclinic system with space group P21/c and the lattice parameters are a = 8.598(5) Å, b = 11.336(2) Å, c = 11.023(2) Å, β = 102.83(1)° and volume = 1047.5(3) Å3. High-resolution X-ray diffraction measurements were performed to analyze the structural perfection of the grown crystals. Thermal analysis shows a sharp endothermic peak at 124 °C due to melting reaction of 2-aminopyridinium trichloroacetate. UV–vis–NIR studies reveal that 2-aminopyridinium trichloroacetate has UV cutoff wavelength at 354 nm. Dielectric studies show that dielectric constant and dielectric loss decreases with increasing frequency and finally it becomes almost a constant at higher frequencies for all temperatures. The negative nonlinear optical parameters of 2-aminopyridinium trichloroacetate were derived by the Z-scan technique.

Research highlights

► Good quality crystals of 2-aminopyridinium trichloroacetate were grown for first time. ► 2-Aminopyridinium trichloroacetate crystal belongs to monoclinic crystal system with space group P21/c. ► 2-Aminopyridinium trichloroacetate crystal exhibits third order nonlinear optical properties. ► 2-Aminopyridinium trichloroacetate is a low dielectric constant material.

Introduction

Extensive studies have been made on the synthesis and crystal growth of nonlinear optical (NLO) materials over the past decade because of their potential applications in the field of telecommunications, optical signal processing, and optical switching. Organic molecules containing conjugate systems have some advantages over inorganic materials because of the possibility of highly enhanced electronic nonlinear optical polarization responses. The basic structure of organic NLO materials is based on the π bond system; due to the overlap of π orbitals, delocalization of electronic charge distribution leads to a high mobility of the electron density. Functionalization of both ends of the π bond system with appropriate electron donor and acceptor groups can enhance the asymmetric electronic distribution in either or both ground and excited states, leading to an increased optical nonlinearity. While the engineering for enhancing second order NLO efficiency is relatively well understood, the need for efficient third order molecules and materials still exists. In particular, the strong delocalization of π electrons in the organic backbone determines a high molecular polarizability and thus third order optical nonlinearity. In general, large hyperpolarizabilities are the result of an optimum combination of various factors such as π delocalization length, donor–acceptor groups, dimensionality and orientation for a given molecular structure [1], [2], [3].

Trichloroacetic acid forms crystalline complexes with amines and amino acids [4], [5]. It is interesting to study the association of trichloroacetic acid with heterocyclic molecule 2-aminopyridine from the crystal engineering viewpoint. We have successfully synthesized the crystalline salt 2-aminopyridinium trichloroacetate (2APTC) and crystals were grown using slow evaporation solution growth technique for the first time. In the complex formation, protonation of pyridine ring nitrogen facilitates hydrogen bonding interaction between trichloroacetate and 2-aminopyridine. In this paper, we report material synthesis, solubility, crystal growth, structural, optical, thermal, dielectrical and mechanical studies and third order nonlinear optical properties of 2APTC.

Section snippets

Material synthesis

Single crystals of 2APTC were grown from saturated aqueous solution containing 2-aminopyridine (SRL, India) and trichloroacetic acid (Merck) in equimolar ratio. After preparing saturated solution of 2-aminopyridine, the proportionate amount of trichloroacetic acid was added slowly with slightly heating the solution for bringing a homogeneous mixture. Precipitate of crystalline substance was obtained with continuous stirring of the solution of pH value 6.1. The purity of synthesized compound was

X-ray crystal structure determination

The unit cell parameters and the crystal structure were determined from single crystal X-ray diffraction data obtained with a Bruker SMART APEXІІ CCD detector diffractometer (graphite monochromated, MoKα = 0.71073 Å). The unit cell parameters of the 2APTC crystal were measured at 293 K. The data were integrated using Bruker SAINT; corrections for absorption and decay were applied using Bruker SAINT. The crystal structure was solved by a direct method with the SHELXS-97 program and refined by the

Conclusions

Single crystals of 2APTC were grown by solution growth technique for first time and its solubility and metastable zone width were determined. X-ray diffraction analysis reveals the molecular arrangements and the formation of hydrogen bonds in the crystal. HRXRD analysis indicates that crystalline perfection of 2APTC crystal is fairly good. Fourier transform infrared spectral analysis was carried out to identify the functional groups in 2APTC. The low values of dielectric constant and dielectric

Acknowledgements

This work, supported by Department of Science and Technology, Government of India under the grant of project ref-SR/FTP/PS-20/2005, is hereby gratefully acknowledged. P.V. Dhanaraj is grateful to Council of Scientific and Industrial Research, Government of India, for the award of Senior Research Fellowship.

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